Thin-film photovoltaic devices can include semiconductor material deposited over a substrate, for example, with a first semiconductor layer serving as a window layer, a second semiconductor layer serving as an absorber layer. The window layer and the absorber layer form a junction where light that passes through the window to the absorber layer is converted to electricity.
A reflector layer, which may be made of zinc telluride, may be provided between the absorber layer and a back contact layer to provide a barrier against minority electron carrier flow toward the back contact layer to minimize recombination with hole carriers at the back contact layer. Specifically, semiconductor materials, like any other solids, have an electronic band structure consisting of a valence band, a conduction band and a band gap separating them. When an electron in the valence band acquires enough energy to jump over the band gap and reach the conduction band, it can flow freely as current. Furthermore, it will also leave behind an electron hole in the valence band that can flow as freely as current. Carrier generation describes processes by which electrons gain energy and move from the valence band to the conduction band, producing two mobile carriers: an electron and a hole; while recombination describes processes by which a conduction band electron loses energy and re-occupies the energy state of an electron hole in the valence band. In a p-type semiconductor material like the absorber layer, electrons are less abundant than holes, hence they are referred to as minority carriers whereas holes are referred to as majority carriers. The reflector layer is made of a semiconductor material with an electron affinity that is lower than that of the absorber layer. The reflector layer therefore repels electron flow toward the reflector layer back toward the absorber layer, thus minimizing recombination at the back contact. This is described in U.S. Provisional Patent Application 61/547,924, entitled “Photovoltaic Device And Method Of Formation,” filed on Oct. 17, 2011.
During manufacture of photovoltaic devices, absorber layers are sometimes subjected to cadmium chloride treatments in order to improve the absorber layers' crystalline quality (e.g., increasing grain (crystallite) size and curing defects in the crystal lattice including defects located at grain boundary areas. Defects in the lattice structure including grain boundaries are sources of carrier recombination, which reduces photovoltaic efficiency. A cadmium chloride treatment includes exposing the absorber layer, which may be made of cadmium telluride, to cadmium chloride and heating the absorber layer to an anneal temperature afterwards. The heat helps the chlorine atoms diffuse preferentially through grain boundary areas in the cadmium telluride (i.e., interfaces where crystal grains of different orientations meet). The chlorine atoms further increase the conductivity of the cadmium telluride film by facilitating re-crystallization and curing of defects. Improvements in conductivity and reduction of defects that cause recombination increase photovoltaic efficiency.
However, cadmium chloride treatments have potential disadvantages. For example, in photovoltaic devices having a reflector layer formed of zinc telluride adjacent to the back contact layer, the zinc telluride reflector layer may react with the cadmium chloride to form zinc chloride and cadmium telluride. The reaction between cadmium chloride and zinc telluride is thermodynamically favorable (i.e., the products of the reaction are at a lower energy than the reactants). This reaction consumes the zinc telluride, which erodes (depletes) the reflector layer and may result in reduced ohmic contact (i.e., a low resistance junction that provides electric current conduction between a metal and a semiconductor) between the reflector layer and the back contact layer over time. Such a reduced ohmic contact may impair the delivery of generated electrical power to external devices.
Thus, there is a need to solve this problem by treating the absorber layer with chloride compounds that do not react with the zinc telluride reflector layer. It is believed that a reaction between such chloride compounds and the zinc telluride reflector layer is not thermodynamically favorable. Erosion of the zinc telluride reflector layer is therefore limited.
Accordingly, treatment of photovoltaic devices with an alternative chloride compound that obviates the effect of the above-mentioned potential problems is desirable.